A common method of defrosting a commercial refrigeration system frosted evaporator is to halt the refrigeration cycle and activate electric heaters in the evaporator. This method is time consuming and often leads to temperature cycling of the refrigerated space. This cycling can drastically affect the life of the product, frequently foodstuff, being cooled in the refrigerated space.
Commercial refrigeration systems which utilize a hot gas defrost cycle have been in use for many years. In one such arrangement, the refrigeration cycle is merely reversed to cause hot vaporous refrigerant from the compressor to cycle in reverse into the evaporator outlet, through the evaporator, out its inlet to the condenser outlet, through the condenser, out its inlet and back to the compressor. The systems have proved to be very inefficient.
Another method of hot gas defrost is illustrated in U.S. Pat. No. 2,770,104--Sweynor, which describes an older system. That system merely bypassed the condenser in the defrost cycle, an arrangement found to be unsuitable for two reasons. Since the temperature of refrigerant in the compressor suction line was too low, it produced some liquid which entered the compressor, ultimately causing compressor damage. Also, the temperature of the vaporous refrigerant delivered to the evaporator during the defrost cycle was found to be too cool to effect rapid defrosting.
The Sweynor improvement added a means of superheating the refrigerant discharged by the compressor and delivered to the evaporator. This heat was provided by electrically heating a tank filled with water through which the compressor discharge line was routed. Since heat was added to the defrosting cycle, this also raised the temperature of the suction refrigerant. This arrangement added an expensive heater, electricity cost, and heater maintenance cost. It also had the unfortunate result of so heating the evaporator inlet refrigerant temperature that a commercial system having many feet of evaporator inlet tubing would experience sufficient tubing growth to distort and break tubing.
More recently, a system which effects evaporator defrosting in a different manner has met with some commercial success. This is disclosed in U.S. Pat. No. 4,102,151--Kramer et al. This patent relates a hot gas defrost system in which vaporous refrigerant discharged from the compressor during the defrost cycle is routed through a tank filled with water, thus transferring heat to the water and desuperheating the refrigerant delivered to the evaporator. The evaporator discharge line is then routed through this water tank only during the defrost cycle to theoretically superheat the compressor suction refrigerant sufficiently to assure complete vaporization.
However, in practice the assignee of the Kramer patent has found that auxiliary heat is needed for the water tank (located outside) to prevent freezing in the winter. This arrangement thus suffers from several of the drawbacks found with the arrangement disclosed in the above Sweynor patent.
There is a need for a hot gas defrost refrigeration system which is simple, inexpensive and does not rely on external sources of heat for operation.